Update README's example

According to current React version, when I use `React.render`, console shows `React.render is deprecated. Please use ReactDOM.render from require('react-dom') instead.`.

So this PR is a tiny update for the README example.

README.md

@@ -21,7 +21,7 @@ var HelloMessage = React.createClass({
   }
 });
 
-React.render(
+ReactDOM.render(
   <HelloMessage name="John" />,
   document.getElementById('container')
 );